Abstract

Background: The high relapse rate in ovarian cancer patients is highly consistent with a cancer stem cell (CSC) model in which rare, inherently chemoresistant CSC are capable of proliferating and differentiating to cause disease. We recently demonstrated ovarian CSC (OvCSC) can be defined by Aldehyde Dehydrogenase (ALDH) enzymatic activity. ALDH expression is increased in chemotherapy resistant OvCSC and ALDH KO can reverse chemotherapy resistance. While ALDH inhibitors exist, they are not isoform specific. We therefore identified and screened numerous putative ALDH inhibitors, based on molecular homology to the known ALDH inhibitor DEAB, and assessed their impact on OvCSC.

Methods and Results: While most ALDH inhibitors, including DEAB, had no impact on CSC, we identified a novel compound, 673A which specifically depletes OvCSC in vitro. Supporting functional depletion of OvCSC, treatment of primary tumor ascites resulted in 4-20 fold decrease tumor sphere formation. ALDH inhibition is highly synergistic with cisplatin both in vitro and in vivo as assessed by cell growth curves and tumor growth. Pre-treatment of tumor cells with 673 significantly reduced tumor in tumor initiation and growth rates. Unlike non-CSC depleting ALDH inhibitors, 673A is a pan-ALDH1A isoform specific inhibitor with IC50<230 nM for ALDH1A1, 1A2, and 1A3. OvCSC Caspase independent cell death was accompanied by negative Annexin V stain and HMB1 translocation to cytoplasm. IF demonstrated nuclear swelling and loss of DNA in a manner consistent with karyolysis and programmed cell necrosis. TEM of treated cells demonstrated clear cellular, nuclear, and organelle swelling and severe vacuolation of cytoplasm consistent with necrosis. Consistent with the induction of programmed cell necrosis, 673A treatment resulted in increased intracellular calcium. Furthermore, the intracellular calcium chelator BAPTA was able to rescue ALDH inhibition-induced necrosis. As RIP1 and RIP3 kinase activity is known to be a major mediator of necroptosis, we next assessed the impact of the RIP1 kinase inhibitor, Nec-1, and down-regulation of RIP1 and RIP3 by siRNA. Inhibition or depletion of RIP Kinase 1 did not prevent 673A mediated necroptosis of OvCSC. 673A initiates necroptotic cell death through translocation of RIPK3 downstream target, MLKL, to membrane and activation of mitochondria fission protein DRP1 and its translocation from cytoplasm to mitochondria.